Similarly, it's a "complete" protein, whereas most vegetables and legumes are missing necessary amino acids.
The downside of beef isn't the "density" of nutrients: the downside is high saturated fat. Chicken breast, though, is similarly high in protein without the saturated fat downside.
In practice, there's no evidence of amino acid deficiency in vegans/vegetarians except ones that restrict even further (potato diet, fruitarians, etc) https://pmc.ncbi.nlm.nih.gov/articles/PMC6893534/
Besides the ever-popular soybean being a complete protein, if you have normal variety in your diet, it's just not something you have to worry about.